1. Field of the Invention
The present invention relates to a knock control apparatus that controls a knock occurring in an internal combustion engine.
2. Background Art
There is a method of detecting a knocking phenomenon occurring in an internal combustion engine using a vibration sensor (hereinafter, referred to as the knock sensor) directly attached to a block of the internal combustion engine. The idea underlying this method is as follows. That is, when a knock occurs while the internal combustion engine is in operation, vibrations in a specific frequency band are known to occur according to a bore diameter of the internal combustion engine or a vibration mode of the knock. Hence, a knock is detected by measuring vibration intensity (hereinafter, referred to as the knock signal) at this specific frequency.
Also, there is a knock control apparatus that suppresses a knock when the knock is detected by correcting spark timing to shift to a retard side and minimizes a torque reduction when a knock is not detected by returning spark timing to an advance side. This apparatus is based on the idea as follows. That is, as a characteristic of the internal combustion engine, it is known that when spark timing is advanced, an output torque of the internal combustion engine is enhanced whereas a knock occurs more readily, and conversely, when spark timing is retarded, an output torque of the internal combustion engine is reduced whereas a knock hardly occurs. Hence, this knock control apparatus corrects spark timing by shifting spark timing to a retard side when a knock is detected and returns spark timing to an advance side when a knock is not detected, thereby controlling the internal combustion engine to operate at knock limit spark timing at which a largest torque is generated while the occurrence of a knock is suppressed.
It should be noted, however, that a knock may not occur even when spark timing is advanced to the extent that a torque reaches a maximum in a case where the internal combustion engine is operating at a low load. The knock control as above is not necessary in such an operating range.
In the knock control apparatus for an internal combustion engine configured as above, a knock determination threshold on the basis of which to determine a knock is generally set using a gain and an offset preliminarily matched to an average value of a knock signal calculated by filter processing or an average value and a standard deviation of a knock signal calculated by filter processing. However, the average value and the standard deviation of a knock signal vary with a change of an operating state of the internal combustion engine. Accordingly, there are problems that it takes a large number of man-hours to match a gain and the like to the average value in response to such a variance and that an erroneous knock detection or omission of a knock detection occurs because the knock determination threshold is not set adequately.
In order to overcome such problems, methods of providing an improvement for an erroneous knock detection and omission of a knock detection are proposed, for example, in Patent Document 1 and Patent Document 2. According to the disclosed methods, the knock determination threshold is set adequately by suppressing variances of an average value and a standard deviation of a knock signal caused by a change of the operating state by applying normalization (standardization or non-dimensionalization) to the knock signal.